Issue 39680: datetime.astimezone() method does not handle invalid local times as required by PEP 495

Let g be a an invalid time in New York spring-forward gap:

>>> g = datetime(2020, 3, 8, 2, 30)

According to PEP 495, conversion of such instance to UTC should return a value that corresponds to a valid local time greater than g, but

>>> print(g.astimezone(timezone.utc).astimezone())
2020-03-08 01:30:00-05:00

Also, conversion of the same instance with fold=1 to UTC and back should produce a lesser time, but

>>> print(g.replace(fold=1).astimezone(timezone.utc).astimezone())
2020-03-08 03:30:00-04:00

Note that conversion to and from timestamp works correctly:

>>> print(datetime.fromtimestamp(g.timestamp()))
2020-03-08 03:30:00
>>> print(datetime.fromtimestamp(g.replace(fold=1).timestamp()))
2020-03-08 01:30:00